The present invention pertains generally to the treatment of aqueous starch compositions with alkaline materials and, in particular, to a method of admixing aqueous granular starch slurries with relatively concentrated aqueous alkaline solutions without pasting the granular starch material and without swelling said granular starch material to such a degree that it becomes essentially unfilterable.
The preparation of etherified starch derivatives in non-gelatinized, granular form has been known and practiced for many, many years. As a specific example, it has long been known to produce hydroxyalkylated (e.g., hydroxyethylated) granular starch derivatives by incorporating an alkaline catalyst (e.g., sodium hydroxide) into an aqueous granular starch slurry in an amount sufficient to promote the etherification reaction between the starch material and an alkylene oxide reactant while at the same time avoiding swelling of the starch to a non-filterable condition. Moreover, the use of alkaline materials such as sodium hydroxide, potassium hydroxide, etc. as a catalyst in other granular starch derivatization reactions and/or for other granular starch treatment purposes (e.g., fat and/or protein removal) is also known in the art. As a means of avoiding unacceptbale swelling or pasting of the granular starch material by the alkaline catalyst, it has been common practice to employ salt swelling inhibitors such as, for example, sodium chloride and sodium sulfate.
In U.S. Pat. No. 3,632,803 to Hjermstad et al. (issued Jan. 4, 1972), it is disclosed that the previously practiced method of adding the alkali solution or alkali-salt solution to the surface of the granular starch slurry (or to the agitator vortex) in an agitated reaction vessel, even when done with the aid of salt swelling inhibitors and at a rate of addition sufficiently slow to avoid localized starch gelatinization, nonetheless results in non-uniform granular starch activation which, in turn, can apparently result in undesired gelatinization or solubilization of the more highly activated starch granules during subsequent etherification of the activated granular starch slurry with monofunctional etherifying agents or during subsequent acid thinning of the etherified starch product. As a solution to such problem, the indicated Hjermstad et al. patent proposed an improved process which involved preactivating the granular starch material by admixing an aqueous slurry thereof with the aqueous alkali solution (or with an aqueous alkali/salt solution) under turbulent flow conditions in an in-line mixer external to the batch reactor to be used for ultimately carrying out the etherification reaction of interest.
In carrying out the above-described improved process, it appears that Hjermstad et al. contemplated that the aqueous alkali or aqueous alkali/salt solution to be admixed with the starch slurry would generally correspond to that which was conventionally employed for addition directly into an agitated batch of the aqueous granular starch slurry and that said aqueous alkali solution would therefore be of relatively limited or dilute alkali concentration (e.g., about 5 weight percent or less in the case of sodium hydroxide on a total aqueous alkali solution weight basis) and would typically contain substantial amounts (e.g., about 20 weight percent on a total aqueous alkali/salt solution weight basis) of conventional salt swelling inhibitors such as sodium chloride, sodium sulfate, etc.,